This invention relates to a malfunction sensing apparatus which can sense malfunctions in a fuel vapor control system for an internal combustion engine.
An internal combustion engine for a vehicle, such as an automobile, is generally supplied with fuel from a fuel tank mounted on the vehicle. When the vehicle is stationary for long periods, fuel vapors are generated by the fuel within the fuel tank. In order to prevent these vapors, which may contain harmful hydrocarbon components, from escaping to the atmosphere and becoming a source of air pollution, modern automobiles are commonly equipped with a fuel vapor control system which adsorbs the fuel vapors from the fuel tank when the engine is off and then supplies the fuel vapors to the engine for combustion when the engine is running.
A typical fuel vapor control system for an automotive vehicle includes a canister containing an adsorbent such as activated charcoal. The canister has an inlet connected to the fuel tank of the vehicle and an outlet connected to the air intake pipe of the engine of the vehicle. When the engine is off, fuel vapors travel through from the fuel tank into the charcoal canister and are adsorbed. When the engine is turned on, the intake manifold vacuum sucks the adsorbed vapors out of the charcoal canister and into the engine for combustion. The charcoal canister generally includes a portion that is open to the atmosphere so that the intake manifold vacuum causes atmospheric air to sweep through the canister and purge the charcoal of the adsorbed fuel vapors.
When there is a malfunction of the fuel vapor control system, such as a breakage of tubing between the fuel tank and the charcoal canister or between the canister and the engine, deterioration of the charcoal canister, or the like, fuel vapors can escape to the atmosphere, thereby defeating the purpose of the fuel vapor control system. Therefore, a malfunction sensing device has been proposed in order to detect such malfunctions and generate a warning to alert a driver of the vehicle of the problem so that he can have the fuel vapor control system repaired. In one malfunction sensing apparatus which has been proposed, the internal pressure of the fuel tank is monitored. During engine operation, if the fuel vapor control system is operating normally, a negative pressure should develop within the fuel tank due to the intake manifold vacuum of the engine, since the fuel tank communicates with the air intake pipe via the fuel vapor control system. In contrast, if there is a leak to the atmosphere or similar problem in the fuel vapor control system, only a very slight negative pressure will be produced in the fuel tank. Therefore, when the pressure in the fuel tank does not fall to a suitable level when the engine is running, it is determined that there is a malfunction in the fuel vapor control system.
However, under certain conditions, such as when the outside air temperature is high, the vapor pressure of the fuel in the fuel tank will be quite high. Therefore, when the engine is running, the presence of the fuel vapor in the fuel tank will prevent the pressure in the fuel tank from exhibiting the decrease indicative of normal operation. As a result, even though the fuel vapor control system is actually functioning normally, a conventional malfunction sensing apparatus will mistakenly determine that it is malfunctioning and will generate a warning, which can cause confusion, trouble, and expense for the driver of the vehicle.